Method of injection molding

ABSTRACT

An injection mold apparatus has a core and cavity halves. One or more gas poppet valves are affixed in the injection mold either in the movable and/or in the stationary halves of the mold. The poppet valves (or poppets) are located to open into a mold portion formed in the cavity between the movable portion of the injection mold and the fixed mold portion when the mold portions are closed. At least one gas line is connected to the poppets and to a gas pressure source and is adapted to open the poppets to the passage of nitrogen or other gas into the mold following the injection of molten plastic into the closed mold halves. Porous metal covers may be used to cover the opening in the mold either the movable and/or stationary halves of the injection mold in lieu of the gas poppet valves, and the gas pressure through the porous metal cover can be used to carry out the molding operation. An injection molding method is also taught using the injection of nitrogen gas into the mold immediately folowing the injection of the liquid plastic.

This application is a continuation of application Ser. No. 08/016,261,filed Feb. 11, 1993, now abandoned, which was a division of applicationSer. No. 07/798,863, filed Nov. 22, 1991, now U.S. Pat. No. 5,273,707,which was a continuation of application Ser. No. 561,157, filed Aug. 1,1990, now abandoned, which was a continuation of application Ser. No.279,849, filed Dec. 5, 1988, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to an injection molding method andinjection mold and especially to an injection mold with gas openings onthe stationary or movable halves of the mold or from both sides forapplying gas under pressure to the plastic mold.

Injection molding machines are used in shaping thermoplasticcompositions. The preheating, plasticizing and shaping are all done inthe injection molding machine where the thermoplastic is heated andplasticized and injected under pressure into the mold by means of ascrewram which forces the material through a nozzle in to the mold. Themolds are formed in two or more components which fit together to form acavity therebetween. The mold is mounted to the injection moldingmachine and has a fixed portion and a moving portion. The mold has amovable portion which is connected to a moving platen of the moldingmachine for opening and closing the mold. When the mold is closed, theheated (plasticized) plastic is injected into the cavity, where it isallowed to cool to sufficiently harden the polymer before opening themold by the moving of the movable portion of the injection mold. Thepolymer part or parts are then ejected from the mold which has formed anarticle of manufacture or a portion of an article of manufacture.

The present invention deals with a molding process in which the movable,stationary or both portions of the mold has gas injection ports fittedto each part of the mold to apply a nitrogen or other gas pressure tothe one side or both sides of the part being molded during the moldingprocess. The gas entry ports are covered either with a porous metalwhich is porous to the flow of the nitrogen gas but will block the entryof the more viscous plasticized polymer into the porous metal holes. Theports may also be sealed with poppet valves which are opened under thegas pressure which is applied sometime after the injection of the moltenplastic into the mold cavity.

Gas and especially nitrogen has been commonly used in injection moldingprocesses to apply a gas pressure through the injection nozzle forincreasing the pressure behind the molten plastic being injected intothe mold cavity. Two prior U.S. patents to Sauer, U.S. Pat. Nos.:4,295,811 and 4,309,380, show a blow molding process applying the gasinto the thermoplastic articles through a porous metal section equippedto the core rod which is inserted into the polymer for expanding thepolymer into a mold. These patents teach a different type of moldingprocess from the present invention by using a porous metal for injectingthe required air to form the blow molded part.

The aim of the present invention is to provide an injection mold andmethod which can apply a gas pressure to the outer surfaces of themolding and can apply the gas to a plurality of locations, at least oneor more in each isolated mold portion, which gas pressure can be thesame or can be varied within each section of the mold.

SUMMARY OF THE INVENTION

An injection molding method and mold apparatus are provided having afixed mold portion having a sprue bushing therethrough and having amovable platen slidably mounted on a plurality of tie bars and having amovable portion of an injection mold attached thereto. At least one gaspoppet valve is mounted to the mold and each poppet valve is located toopen into one or more separate isolated mold portions formed between themovable portion of the injection mold and the fixed portion of theinjection mold when the mold portions are closed. Alternatively, gasbores can have openings in each of the separate isolated molds portionbetween the movable portion of the injection mold and the fixed portionof the injection mold when the mold portions are closed with the borescovered with a porous metal of predetermined porosity. At least one gasline is connected to the poppet valves, or to one end of the end borehaving the porous metal cover. Each gas line can be connected at theother end to a pressure source adapted to pressurize each poppet valve(or through each porous metal cover) to open the valve to the passage ofgas, such as nitrogen, into the mold following the injection of plasticinto the closed mold halves. Gas pressure can be applied to each poppetvalve, either simultaneously or intermittently. Each poppet valve isspring loaded with a predetermined pressure to close the valve, and apredetermined fluid pressure placed on the back of the valve overcomesthe pressure of the spring. The poppet is opened and similarly, the gasmay be controlled against and through the porous metal discs whichprevent the flow of the plastics back into the bores when the injectionmold is being filled with a melted plastic.

A method of forming an injection molded part includes the steps ofattaching mold halves having a cavity therein onto an injection moldingmachine and attaching a gas pressure line to the mold. A predeterminedquantity of melted thermoplastic polymer having a lesser volume than themold cavity is injected into the mold. A gas such as nitrogen isinjected under pressure into the mold cavity through one or moreopenings into the mold while the melted thermoplastic polymer is fluidto completely fill the mold cavity under a predetermined pressure topressurize the mold and thermoplastic polymer in the mold. Thethermoplastic polymer is then cooled and the molded part is ejected fromthe mold. Typically, a plurality of gas lines pressurize the interior ofthe mold from a plurality of different positions from the same or adifferent gas source through poppet valves or porous metal covers.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention will beapparent from the written description and the drawings in which:

FIG. 1 is a sectional view taken through a portion of an injection moldin a closed position in accordance with the present invention;

FIG. 1a is an enlarged view of the portion circled in FIG. 1;

FIG. 2 is a sectional view taken through an injection mold havingseparate isolated mold portions formed in the mold;

FIG. 3 is a sectional view taken through one poppet valve and isolatedmold portion in accordance with the present invention;

FIG. 4 is a side elevation of an injection mold having isolated moldportions;

FIG. 5 is a sectional view of an alternate embodiment of a mold usingporous metal covers; and

FIG. 6 is a diagrammatic view of a gas supply system.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawings and especially to FIG. 1, the molding portionof an injection mold and machine 10 has a stationary platen 11 and amoving platen 12 sliding on a plurality of tie bars 13. A clamping ram14 drives the moving platen 12 to open and close the mold. A fixed moldhalf 15 is attached to the fixed platen 11 with bolts 16 while a movingmold portion 17 is attached with bolts 18 to mold members 20 which arein turn attached with bolts 21 to the moving platen 12. A steel plate 22is attached between the attaching members 20 and the moving mold portion17. When the moving mold portion 17 is driven by the clamping ram 14 toclosed the mold as shown in FIG. 1, a molding cavity 23 is formedbetween the mold halves 15 and 17. The fixed platen 11 has a plasticinjection nozzle 24 connected to an injection storage space 25 which hasthe molten plastic or polymer driven by a screw plunger 26 into thecavity 23 of the closed injection mold. A location ring 95 is formedaround the opening 29 passing through the mold path 15. Tie bars 13 areheld by tie bar nuts 27 to the fixed platen 11.

The movable mold portion 17 has a plurality of poppet valves 28. Eachpoppet valve has a valve member 30 having the valve element 31 on oneend to close a bore 32 entering into the cavity 23 through the movablemold portion. Each valve member 30 has a head portion 39 riding in acavity 33 and having a spring 34 therein biasing against the head 39 tokeep the poppet valve member 30 in a closed position to prevent theentry of a fluid through the valve element 31 and into the cavity 23 andthe reverse is also true to keep the injected plastic from going intoarea 54 of FIG. 3. A gas line 35 connects to the side of the bore 32.Similarly, a second poppet valve 36 has a gas line 37 connecting theretoand to a third poppet valve 38. The opening in each poppet valve isconnected into an isolated portion of the mold as shown in FIG. 2. Eachpoppet valve 28 is opened by applying pressure in the gas line 35 or 37until sufficient pressure is reached to overcome the pressure within theclosed mold cavity 23 and to overcome the spring bias 33 to allownitrogen gas to enter into the mold cavity 23 on the opposite side fromwhere the polymer has been injected through the nozzle 24. It shouldhowever be clear that the gas can be applied to either or both sides ofthe mold. Different pressures can be utilized in different gas lines 37and 35, or alternatively, all of the gas lines can be simultaneouslypressurized with the same gas pressure or can be pressurized withdifferent pressures. Each opening of each poppet valve 28 is placedaround the mold cavity in isolated sections of the mold which wouldotherwise be blocked off from the entry of gas. For instance, in FIG. 2,a cavity portion 40 might have a cavity wall 41 and the side wall 42forming an isolated portion 40 while the walls 41 and 43 form anisolated cavity portion 44 and similarly isolated portions 45 and 46 canbe formed within the same mold. Each cavity portion with the walls 41and 43 blocks the gas that enters into the isolated mold portion frompassing to the next isolated mold portion.

In operation, the ram 14 opens and closes the mold halves 17 and 15.Once the mold is closed a molten thermoplastic polymer 49 is injectedthrough the nozzle 24 by the operation of the screw ram 26 to drive thepolymer into the cavity 23. When the plastic injection, is completed,filling but not packing out the mold, i.e. the mold is 90 to 95% full,and pressure is allowed through directional valve or valves 82 to lines35 and 37 to open the poppet valves 28 to allow the gas to enter intothe cavity 23. The gas forces the polymer against the other side of themold to provide a large part of uniform strength characteristics as wellas one that is easier to remove from the mold once the movable portionof the mold 17 is pulled away to open up the mold halves 15 and 17. Aneven film of gas under pressure forms an open area on one side of themold cavity and results in the easy ejection of the part after themolding operation.

In FIG. 3, a portion of a mold half 15 has a movable mold portion 17mounted adjacent thereto enclosing the mold to form a portion of thecavity 47. A gas line 48 is connected through the movable mold portion17 into a poppet valve 50 having a spring 51 driving against a head 57to keep the poppet valve element 53 closed. The gas line 48 enters intoan enlarged bore area 54 around the stem 55. Sufficient gas pressure isapplied to the valve to overcome the spring 51 so that when the pressureis applied against the surface of the valve element 53 it will overcomethe pressure in the cavity 47 to allow the gas in the line 48 to passinto the cavity 47. Since a molten plastic 56 has been injected into thecavity mold and gas through the poppet valve has been injected into theother side, the pressurized gas 57 forms a substantially even film ofgas across the whole surface of the molding between the mold half 17 andthe molten polymer 56 all around the portion of the molded part butblocked where the mold comes together at 60 and 61 to isolate thatportion 47 of the cavity. The several poppet valves, as illustrated inFIG. 1, are connected directly to each isolated portion of the moldcavity. An O-ring seal 59 seals around each poppet valve stem 55 to sealagainst the escape of gas thereby.

As seen in FIG. 4, a mold cavity 62 is divided into four quadrants 63each having a poppet valve 64 therein for the escape of gas 65thereinto. A surrounding wall 66 is also formed in the cavity which isbroken into four isolated mold portions by the crossing walls 67. Itwould be clear that this is a simplified elevation of a mold used toillustrate the principal of the present invention.

In FIG. 5 a mold cavity 70 is illustrated in a modified embodiment inwhich a movable portion of a mold cavity has gas lines 71 feedingdirectly through open bores to the cavity which is covered with a porousmetal 72. Thus, a nitrogen gas may be fed through the ine 71 and willreadily pass through the porous metal cover 72 and into the cavity 70for each isolated portion of the mold. The molten polymer is a viscousliquid and is blocked by the porous metal 72 from getting into the bore71.

In operation a predetermined pressure has to be applied to the line 71at the appropriate synchronized time to apply the gas to each sector ofthe inside of the mold.

turning to FIG. 6 a diagrammatic view of a mold in accordance with thepresent invention is illustrated having a tank of nitrogen 75 feeding agas line 76 through a control valve 77. The gas can be fed through aplurality of lines 78 and 80 to different portions of the mold and mayhave pressure gages 81 and cut off valves 82. The gas is fed through theline 83 into a poppet valve 84 and into a mold cavity 85. Alternatively,the gas can be fed through a gas line 86 through a porous metal 87 intothe mold cavity 85. Each poppet valve or porous valve cover 87 isnormally placed within an isolated portion of the mold cavity so thatgas is applied to all isolated portions of the mold cavity during themolding process. A gas release line 88 is fed past a valve 90 to ventthe gas out the line 91.

A method of forming an injection molded part includes attaching a moldhaving a cavity formed therein with a fixed mold portion and movablemold portion to an injection molding machine including the step ofattaching at least one gas pressure line to the mold. The methodincludes injecting a predetermined quantity of melted thermoplasticpolymer into the mold in which the predetermined quantity of meltedthermoplastic is a lesser volume than the mold cavity. Then injecting agas, especially a nitrogen gas, under pressure into the mold cavitythrough at least one gas pressure line while the melted thermoplasticpolymer is still fluid, to completely fill the mold cavity under apredetermined pressure to pressurize the thermoplastic polymer. Themelted thermoplastic polymer is then cooled to harden the polymer in themold and the part is then ejected from the mold. In the preferredembodiment, one or more gas pressure lines are attached to the mold andthe gas injected under pressure into the mold is injected through theplurality of the gas pressure lines. The step of attaching a pluralityof gas pressure lines to the mold can attach the pressure lines througheither the fixed or the movable portion of the mold or both but, in theillustrated embodiment, the gas lines are attached to the movableportion of the mold especially to mold portions opening in differentisolated cavity portions of the mold. Gas lines may be attached throughpoppet valves which are spring loaded to open only when a predeterminedgas pressure is applied to the gas lines. Alternatively, the gas linescan be attached to the mold adjacent the opening through the mold whichhas a porous metal cover over each opening into the mold cavity whichallows the gas under pressure to be applied to the mold cavity whileblocking the melted thermoplastic from entering the gas openings intothe mold.

It should be clear at this time that an injection molding method andapparatus has been provided which provides gas or fluid underpredetermined pressure to at least one inlet into a mold through eithera porous metal cover or a poppet valve to apply the same or differentpressures to different isolated portions of the mold. It should however,be clear that the present invention is not intended to be limited to theforms shown which are to be considered illustrative rather thanrestrictive.

I claim:
 1. A method of forming an injection molded part, comprising thesteps of:attaching a mold having a part defining cavity formed by afixed portion and a movable portion to an injection molding machineincluding the step of attaching at least one gas pressure line to saidmold; injecting a predetermined quantity of molten thermoplastic polymerinto said cavity, said predetermined quantity of thermoplastic polymerbeing of a lesser volume than the volume of said cavity; introducing agas under a predetermined pressure into said cavity through said one gaspressure line while said molten thermoplastic polymer is fluid topressurize said thermoplastic polymer and to completely force saidmolten thermoplastic polymer against only one said mold portions;cooling said thermoplastic polymer in said cavity; and ejecting the partfrom said mold.
 2. A method of forming an injection molded part inaccordance with claim 1 in which the step of attaching at least one gaspressure line to said mold includes attaching a plurality of gaspressure lines to said mold and the step of injecting a gas underpressure into said mold cavity includes injecting a gas through aplurality of gas pressure lines.
 3. A method of forming an injectionmolded part in accordance with claim 2 in which the step of attaching aplurality of gas pressure lines to said mold attaches said plurality ofgas lines to said mold movable portion.
 4. A method of forming aninjection molded part in accordance with claim 3 in which the step ofattaching a plurality of gas pressure lines to said mold attaches eachgas pressure line through a poppet valve opened by a predetermined gaspressure on said gas line.
 5. A method of forming an injection moldedpart in accordance with claim 3 in which the step of attaching aplurality of gas pressure lines to said mold attaches each gas pressureline through a porous metal cover over an opening into said mold cavity.6. A method of forming an injection molded part in accordance with claim2 in which the step of of injecting a gas under pressure into said moldcavity includes injecting a gas through a plurality of gas pressurelines, at least one of said gas pressure lines having a pressuredifferent from the other gas pressure lines.
 7. A method of forming aninjection molded part in accordance with claim 2 in which the step of ofinjecting a gas under pressure into said mold cavity includes injectinga gas through a plurality of gas pressure lines under the same gaspressure in each gas pressure line.
 8. A method of forming an injectionmolded part in accordance with claim 1 including the step of injectinggas under pressure in said mold through said one gas pressure line aftercooling said thermoplastic polymer in said mold whereby said gas aids inthe ejecting of a part from said mold.
 9. The method as claimed in claim1 wherein,said gas introducing step causes said polymer to be forcedagainst said one mold portion and said gas forming an open cavity in aside of said other mold portion, said open cavity being filledcompletely by said gas.
 10. A method of forming an injection moldedarticle comprising the steps of:attaching a mold system having a moldcavity formed in part by article defining surfaces to an injectionmolding machine including the step of attaching at least one gaspressure line to one side of said mold cavity; injecting a predeterminedquantity of molten thermoplastic polymer into said mold cavity, saidpredetermined quantity of thermoplastic polymer being of a lesser volumethan the volume of said cavity; introducing a gas under pressure onlyinto said mold cavity and through said one gas pressure line while saidthermoplastic polymer is fluid to completely fill said mold cavity witha gas cushion to pressurize said molten thermoplastic polymer from saidone side of the mold cavity only towards the opposite side of the cavityfor retaining the thermoplastic against the article defining surfaces;cooling said thermoplastic polymer in said mold; and ejecting the partfrom said mold.
 11. The method as claimed in claim 10 wherein,said moldcavity is formed by a cavity surface interrupted by chambers andcomprises the article defining surfaces forming an outer appearancesurface of the article, and said injecting step includes introducingmolten thermoplastic into said chambers whereby to form an articlehaving an interior surface including bosses and ribs and said outerappearance surface essentially smooth and free of sink marks.
 12. Amethod of producing a plastic article, the steps of the methodcomprising:injecting into a mold cavity having first and second oppositesides a predetermined quantity of molten thermoplastic, said quantitybeing less than the volume of said mold cavity, and said mold cavityincluding cavity areas for defining a resultant shape of a moldinghaving a smooth appearance side essentially free of sink marks and anopposite side including ribs and/or bosses, introducing pressurized gasinto the first side of the mold cavity only and prior to cooling of thethermoplastic such that the gas forces the thermoplastic away from thefirst side of the mold cavity towards the second side of the cavityforming the appearance side of the article, and maintaining the pressureof the gas introduced into the cavity until the molding can sustain theshape dictated by the mold cavity areas.